I interpreted it to mean that if you tolerate deviant behavior, then you'll likely tolerate even more (increasingly) deviant behavior, but just how much you'll tolerate is unknown, until it's too late and things are out of hand.
Seems to me that tolerating "deviant behavior" like abortion and gay marriage can have the same effect. How much of those behaviors you'll tolerate can also be unknown until it is too late and things are out of hand.